1. Field of the Invention
The present invention relates to the applications of co-coped n-type zinc oxide thin film as a chemically stable and low work function cathode to Organic Light Emissive Diode (OLED) display, Field Emission Display (FED) and vacuum microelectronic display devices.
2. Description of Related Art
Display devices are a fast growing segment of the electronics market. Historically, commercially viable displays have relied on cathode ray tube (CRT) and liquid crystal display (LCD) technologies due to reliability and affordability. CRT technology is mature and able to achieve high resolution, high luminance (brightness), low cost and long life. Unfortunately, CRT displays require high operating voltages and are too heavy for portable applications. CRTs also have a large bulky form factor. More recently, flat panel LCDs have gained acceptance in many applications since they operate at power levels compatible with battery operation, are lightweight and have a thin form factor. LCD panels either reflect or transmit light so an external light source is required. LCD panels also have a limited viewing angle so the user may not be able to see the displayed information from an oblique viewing angle. Although the viewing angle for LCD displays has improved over the years, they are still inferior compared to CRTs and other emissive display technologies. Another weakness of LCD displays is that the liquid crystal material response to a stimulus is intrinsically slow at low temperatures. Thus, LCD displays are a poor choice for portable, automotive or military applications where operation at extreme low temperature may be required. Accordingly, there is great need for an inexpensive low-power display technology that exhibits real-time imaging capability in the flat panel form factor over an extended operating temperature range.
There are a number of alternative technologies that offer the luminous efficiency and resolution of a CRT and the flat panel form factor of an LCD that are compatible with portable applications. Light emissive devices have a form factor of an LCD display but are not dependent on external light sources. Emissive devices also have the wide viewing angle of a CRT and will operate over an extended temperature range. Two examples of emissive devices are the organic light emitting diode (OLED) display devices and the field effect display devices. Emissive display devices are lightweight and capable of projecting video rate images with high contrast ratio over an extended temperature range. Emissive displays hold great promise as an alternative to LCDs because they have superior viewing angle characteristics and high video rates. Moreover, unlike LCDs, the response rate of emissive displays is not affected by a low ambient operating temperature.
FIG. 1 illustrates a portion of a prior art OLED device 10. Device 10 has an opaque cathode electrode 12 in a spaced-apart arrangement with a transparent anode electrode 14 deposited on a transparent substrate 18. An organic emissive medium 20 is sandwiched between cathode electrode 12 and an anode electrode 14 with pixels defined wherever the cathode electrode overlaps the anode electrode. Layer 23 may be applied over cathode electrode 12 to encapsulate device 10 and protect the device. When electrons injected into medium 20 from the cathode combine with holes injected at the anode, light, denoted as hxcexd and illustrated as downwardly projected arrows, is generated and transmitted through transparent anode 14 and substrate 18.
The cathode electrode 12 is usually an opaque reflective low work function metal such as an alkaline earth metal or reactive metal alloy. Examples of prior art cathode electrodes include calcium, magnesium/silver, or aluminum/lithium. Typically, the anode electrode 14 is a high work function thin film of transparent indium tin oxide (ITO). The phrase xe2x80x9cwork functionxe2x80x9d refers to the energy difference, in electron volts (eV), between a free electron and an electron at the Fermi level of the material. The phrase xe2x80x9cFermi levelxe2x80x9d indicates the energy level at which the probability that a state of energy is occupied is equal to one half. To minimize the energy barriers, the work function of the cathode needs to be low so that the Fermi level closely matches the energy level of the lowest unoccupied molecular orbital (LUMO) of the organic medium. Similarly, the work function of the anode needs to closely match the energy level of the highest occupied molecular orbital (HOMO) of the organic medium. Since ITO is the material of choice for the transparent anode, prior art research has focused on use of alkaline earth metal cathodes having a low work function to achieve device efficiency. However, alkaline earth metals are extremely reactive and are not transparent.
A significant problem with prior art displays is that the interface between the electrodes and the emissive medium creates energy barriers that must be overcome before charge can be injected into the medium. Where the energy barrier at one electrode is much greater than at the other electrode, the supply voltage must be sufficient to overcome the larger barrier thereby increasing the power that must be supplied to the device. FIG. 2 illustrates the potential energy diagram of the prior art OLED device illustrated in FIG. 1. As indicated, the Fermi level of the ITO (work function is about 4.7 eV as indicated at 22) is above the HOMO energy level of the organic medium. For example, since the HOMO is about 4.9 eV for the MEH-PPV organic polymer, energy is required to inject holes, represented by h+, over the potential energy barrier into the organic medium. Further, the Fermi level of the metal cathode (about 3 eV to 4 eV for typical alkaline earth metals as indicated at 24) is below the LUMO of the medium (about 2.8 eV), so energy is also required to inject electrons, represented by exe2x88x92, into medium 20. Thus, the operating voltage must be sufficient to overcome the potential barriers to inject both electrons and holes into the medium before light will be generated. In many prior art OLED devices, the imbalance in the ratio of hole injection and electron injection generates heat dissipation. Such heating contributes to degradation of the medium and low efficiency of the OLED device.
The use of ITO as the transparent electrode (anode) and a reactive low work function metal as the cathode also constrains OLED device design to the traditional architecture illustrated in FIG. 1. One example is a miniature OLED display integrated monolithically with the driving circuit on a silicon substrate. This device requires that the opaque reactive metal cathode (Ca, Mg) be deposited on the silicon backplane and the ITO anode to be deposited on the organic emissive medium. However, the reactive cathode readily oxidizes and may degrade the organic medium interface. Also, reactive metals are not compatible with the semiconductor processing technology and can degrade driving circuits on silicon substrates. Accordingly, a complicated semiconductor manufacturing process involving barrier layers is required to combine the prior art OLED display device with electronic elements on a common substrate. Clearly, what is needed is an OLED device with a design and a set component materials that may be processed and integrated on a common silicon substrate together with control circuits. It is desirable to have a transparent and stable cathode material with a low work function and a low processing temperature, so it can be used as a substitute for the reactive metal cathode in the prior art.
Field emission display (FED) devices represent another type of emissive display that is similar to traditional CRT display technology in that an independently addressable stream of electrons excites each pixel. Where the CRT uses a single electron source to sweep a single beam of electrons across the back of a phosphor screen, FED devices incorporate an array of emitters (the cathodes), each of which ejects a stream of electrons in the presence of an applied electric field to excite a corresponding pixel on an opposing display surface (the anode). By eliminating the single electron gun and the need to sweep the beam across the screen of the CRT display, the depth and, accordingly, the form factor of the FED display can be significantly reduced.
Each emitter in a FED device has a pyramidal or a conical top terminating in a sharp point coupled at the base to a cathode electrode. A control gate resides just above each emitter or group of emitters. Applying a voltage between each individually addressable gate and cathode electrode results in a steam of electrons, which excites a phosphor at the anode, thus resulting in visible light being generated. The anode electrode sets the bias voltage to attract the electron stream but need not be otherwise addressable. Unfortunately, prior art FEDs have an unacceptably short operational lifetime and require high operating voltages due to various material limitations related to the emitters. Most of the prior art FEDs have emitters that are made from materials such as silicon and refractory metals. Consequently, they have high work functions and require high operating voltages to generate sufficient field emitting electrons. Furthermore, an insulating layer of oxide gradually forms on the emitter surface, increasing its work function and acting as a barrier to electron emission which results in decreasing luminance and uneven brightness of the displayed image over the life of the display. Similar technical issues also impede the performance of vacuum microelectronics devices that mimic various types of vacuum tube electronic devices with individually gated solid-state micro electron emitters. Therefore, both FED and vacuum microelectronic devices will benefit from having an oxidation resistant, low work function electron emitter.
The present invention relates the use of co-doped zinc oxide (n-type) as the cathode of organic light emitting diode (OLED) displays, field emission displays (FEDs) and vacuum microelectronic devices. The invention overcomes the limitations in the prior art described above and other limitations in the prior art that will become apparent upon reading and understanding this specification. The present invention maximizes device efficiency, improves device reliability and extends the flexibility to new device architectures.
In one preferred embodiment, a co-doped zinc oxide film is deposited on a glass substrate. An emissive organic medium is sandwiched between the co-doped (n-type) zinc oxide cathode and an anode made with high work function metals such as gold (Au), platinum (Pt) or nickel (Ni). Light transmits through the zinc oxide cathode and the glass substrate. This device uses a low work function cathode and high work function anode to minimize the electron injection and hole injection energy barrier to maximize device efficiency.
In another preferred embodiment, a bi-directional OLED display has an emissive organic medium sandwiched between a transparent indium tin oxide (ITO) anode and a transparent co-doped zinc oxide cathode. Since both electrodes are transparent, light emits bi-directionally from both the top and the bottom of the device. The co-doped zinc oxide cathode has a low work function that minimizes the potential energy barrier to inject electrons from the cathode into the organic medium so the display device operates more efficiently. Since the co-doped zinc oxide thin film can be deposited at a low temperature, it can be deposited on the organic emissive medium without degrading the medium. This low temperature deposition process is preferred to fabricate any OLED device where the transparent ZnO cathode is not deposited on a glass substrate.
In another preferred embodiment of an emissive display device, the anode is a high work function metal, such as platinum (Pt) or gold (Au). The reflective, high work function metal anode minimizes the potential barrier to inject holes into the organic medium and provides an improved unidirectional display. The non-reactive metal anode may be deposited on silicon or another substrate with a co-doped zinc oxide as the transparent electrode (cathode) deposited and patterned on top of an organic layer. Since both electrodes are non-reactive, an emissive display device may be combined with control electronics on a single substrate. This embodiment is particularly useful for miniature display applications. Miniature flat-panel displays on silicon permit the integration of display drive electronics with the display on a common substrate. This type of miniature display provides high resolution and low cost particularly desired for wireless or mobile applications where low power and high efficiency can minimize weight and increase battery life.
The low work function and chemical robustness of the co-doped zinc oxide electrode also hold application as a field emitting material for FEDs and vacuum microelectronic devices. The present invention provides improved resistance to cathode oxidation common in prior art displays. The invention also provides high operation efficiency because of the low work function of the material.
In one embodiment, a thin film of co-doped zinc oxide is deposited on the emitter surface of FED devices or vacuum microelectronic devices. This coating minimizes performance degradation induced by surface oxidation of the emitters and enhances electron emission efficiency due to the low work function of this material.
Another embodiment describes a field emission light source. In this configuration, a plurality of transparent zinc oxide emitters is fabricated on a transparent substrate and a phosphor layer is applied to a metal anode. An electric field is applied between the emitter and the phosphor to accelerate electron streams toward the anode and bombard onto the phosphor. Light generated on the phosphor layer will transmit through the transparent cathode. In this device geometry, heat generated during the interaction between the electron streams and phosphor is dissipated in the metal anode. The high thermal conductivity of the metal anode effectively removes the heat and enables high power operation to result in high light intensity output.